1. Field of the Invention
The present invention relates to a sheet feeding apparatus configured to feed sheets one by one from a repository which stores a plurality of sheets, and to an image forming apparatus, such as a printer or a copying machine, having such a sheet feeding apparatus.
2. Description of the Related Art
A conventional sheet feeding apparatus in an image forming apparatus, such as a printer or a copying machine, is configured to separate sheets of paper such as a coated paper, which is difficult to separate, and to feed each sheet to an image forming unit. Such a sheet feeding apparatus employs an air sheet-feeding method. The air sheet-feeding type sheet feeding apparatus separates sheets by blowing air against an edge portion of a sheet stack stored in a repository to allow a sheet placed on the top of the sheet stack to float in the air. Then, the sheet feeding apparatus suctions the floated top-placed sheet with a negative pressure to the surface of a suction conveyance belt disposed above the sheet stack and conveys the sheet suctioned to the suction conveyance belt. This method is discussed in U.S. Pat. No. 5,645,274.
In the air sheet-feeding type sheet feeding apparatus, a suction duct is disposed on an inner side of an endless suction conveyance belt. A fan is provided to generate a negative pressure in the suction duct. Accordingly, a sheet is suctioned via a suction hole formed through the suction conveyance belt. The suction conveyance belt having the sheet suctioned thereto rotates to convey the sheet.
When such an air sheet-feeding type sheet feeding apparatus is used, it is necessary to appropriately adjust the level of negative pressure in the suction duct. In this regard, a shutter (or valve) is disposed between the fan and the suction duct to adjust the negative pressure in the suction duct by closing and opening the shutter.
The shutter is opened to reduce the internal pressure of the suction duct to a negative pressure in order to suction a sheet to the suction conveyance belt. When a sheet is suctioned and conveyed by the suction conveyance belt, the shutter is closed after a leading edge of the conveyed sheet has reached a conveyance roller disposed on a downstream side of the belt. Then, the rotation of the suction conveyance belt is stopped to prevent the next sheet from being suctioned and conveyed by the suction conveyance belt.
Japanese Patent Application Laid-Open No. 06-278888 discusses a sheet feeding apparatus including a shield belt having a portion in which a hole in communication with the suction duct is formed and a portion in which no holes are formed. The sheet feeding apparatus adjusts a negative pressure in the suction duct by rotating the shield belt integrally with the suction conveyance belt.
In the conventional sheet feeding apparatus discussed in Japanese Patent Application Laid-Open No. 06-278888, in separating sheets by blowing air against a sheet stack with a lower separation type sheet feeding apparatus, a sheet placed on the bottom of the sheet stack is suctioned to the suction conveyance belt in a state in which the holeless portion is externally directed so that the hole of the shield belt does not face the air. Subsequently, in conveying a sheet by rotating the suction conveyance belt, the shield belt is also rotated so that a surface on which the hole is formed is externally directed to cause the inside of the suction duct to communicate with the outside of the suction duct. That is, the internal negative pressure of the suction duct is controlled according to the rotation of the suction conveyance belt.
However, the conventional sheet feeding apparatus configured to adjust a negative pressure by opening and closing the shutter has the following problems.
Even when the leading edge of a sheet reaches a downstream side conveyance roller and thus a suction operation utilizing a negative pressure is stopped by closing the shutter, the suction hole formed in the suction conveyance belt is closed by the sheet which is being conveyed. Thus, the negative pressure remains in the suction duct. The sheet which is being conveyed is suctioned to the stopped suction conveyance belt. Accordingly, the conveyance roller disposed on the downstream side conveys the sheet suctioned to the suction conveyance belt. Accordingly, a large load is applied to a conveyance motor that drives the conveyance roller disposed on the downstream side. Thus, the conveyance roller cannot stably convey sheets. Accordingly, a skewed conveyance of sheets or jamming of sheets can occur. Furthermore, a sheet is pulled by both the suction conveyance belt and the conveyance roller. Thus, in the case of using a thin sheet whose stiffness is low, wrinkles can be caused on the sheet.
In the case of using a shield belt having a portion in which a hole for communicating with the suction duct is formed and a holeless portion, the shield belt and the suction conveyance belt are driven by the same drive roller. Thus, the apparatus alternately repeats maintaining of the negative pressure in the suction duct using the shield belt and releasing of the inside of the suction duct. Accordingly, the suction conveyance belt alternately repeats suctioning of the sheet utilizing the negative pressure and non-suctioning of the sheet due to absence of the negative pressure. Accordingly, the conveyance of sheets becomes unstable.